It is known that, in the natural state, wood or the wood fibers which are in contact with a humid atmosphere, tend to be logged with water, going as far as absorbing it up to 100% of their weight. Such water absorption is accompanied, on the one hand, by a swelling and, on the other hand, by a loss of the mechanical qualities and qualities of cohesion of the material which, in certain cases, can go as far as an advanced disintegration thereof. This is why the habit is taken, before every step of machining wood, of effecting a drying step which, by eliminating the water therefrom, improves its dimensional stability.
Although the drying step enables the water to be eliminated from the wood, it in no way modifies the hydrophilic nature thereof, with the result that it is again likely to reabsorb the water eliminated during drying when it is again in a humid atmosphere.
In order to decrease the hydrophilic character of the natural wood and thus to give it a long-lasting dimensional stability, different techniques of heat treatment at high temperature have been proposed.
Among these techniques, it has been proposed to subject the natural wood to different steps of treatment including in particular a drying in open circuit followed by heating and maintenance at a temperature included between about 220.degree. C. and 300.degree. C. for a determined period. Such a technique of treatment, called curing, makes it possible to give the wood both a hydrophobic character and an excellent dimensional stability.
However, it has been ascertained that the operation of curing had to be carried out with the greatest rigour, on pain of decreasing the mechanical characteristics of the wood treated. In fact, it is known that this curing step has for its object to partly destroy the hemicellulose of the wood without being detrimental to the structure thereof, in other words without destroying the lignin.
Under these conditions, one of the major difficulties encountered during the curing treatment consists in determining the time during which the curing temperature must be maintained in order to destroy the hemicellulose without significantly destroying the lignin.